An photoacoustic tomography (hereinafter may be referred to as PAT) apparatus is known as one of apparatuses for visualizing in-vivo information. In the measurement using a PAT apparatus, a tomographic image can be obtained by, measuring the intensity and the generation time of a photoacoustic signal emitted from a substance (optical absorber) that absorbs the light in an object to be measured when the measured object is irradiated with a light, and computing a distribution of the substance in the object.
Any substance can be used as an optical absorber, so long as the substance absorbs a light and emits an acoustic wave in a living body. For example, a blood vessel, a malignant tumor, or the like in the human body can be used as an optical absorber. In addition, a molecular probe such as indocyanine green (hereinafter may be referred to as ICG) can be introduced into the body and used as a contrast agent. ICG is a safe substance that has been approved to be administered into the body. Since ICG sufficiently absorbs a light in the near-infrared wavelength range, which has little influence on the irradiated human body and is highly permeable to a living body, ICG can be suitably used as a contrast agent in a PAT apparatus.
Furthermore, ICG is characterized by emitting fluorescence when excited by a light in the near-infrared wavelength range. ICG can also be used as a fluorescent contrast agent by utilizing this characteristic.
Meanwhile, due to its high degradability in a water, ICG administered as a contrast agent has a problem that it is difficult to be accumulated at a site for measurement.
As a method for solving this problem, a technique to entrap ICG at a high concentration in a particle has been developed. For example, the Journal of Photochemistry and Photobiology B: Biology, 74 (2004) 29-38 (hereinafter referred to as NPL 1) discloses an ICG-containing poly(lactide-co-glycolide) (PLGA) that is obtained by an emulsification solvent diffusion method using polyvinyl alcohol (PVA) as a surfactant. Furthermore, International Patent Publication WO 2010/018216 (hereinafter referred to as PTL 1) describes a nanoemulsion having an oil phase containing ICG covered with a surfactant.